The encrypted version of the file will by default be named foo.txt.gpg. You can modify that behavior using the --output (-o) option.

Decrypt the encrypted file. You’ll be asked to provide the passphrase you used when generating your private key. If you don’t use the --output option, the contents of the encrypted file will be sent to standard output.

$ gpg --output foo.txt --decrypt foo.txt.gpg

If foo.txt is a file that you think you’ll want to edit and/or view on a regular basis, you might consider using make to reduce the amount of typing you’ll have to do. Here’s an example Makefile (to use it, you’ll need to make sure that the leading whitespace in the targets is composed of Tabs, not ordinary spaces).